This invention relates to optical disc apparatus, and in particular, to a means for accessing information into and out of optical disc media.
Optical disc technology is currently gaining increasing importance in the field of information storage and retrieval primarily due to greater storage capacity of the discs and the potential for less costly apparatus than standard magnetic disc technology.
Typically, the optical disc includes one or more layers of suitable materials (such as bismuth or selenium) in which pits are formed along a large number of tracks concentric with the center of the disc. The presence or absence of a pit, as well as the length of a pit, can represent information in either digital or analog form. This pit structure can be formed by the light from a powerful "write" laser. Since the pits locally modify the reflectivity of the disc medium, the information can be retrieved by the light from a "read" laser. In addition, a "tracking" laser may be used to determine the position of the read/write head radially and azimuthally with respect to the disc. The lasers are usually included in the read/write/tracking head or closely spaced from the head. A beam splitter or mirror directs the light from a laser/lens combination to a lens which focuses the beam onto a spot on a track of the disc. Where the beam is used for read-out, the reflected light is directed to one or more photo-detectors so that as the disc rotates, the information along the track is reproduced in electrical form. Motion of the head along a radius allows access to any one of the tracks.
One problem associated with such apparatus is that the laser is subjected to vibration and shock during operation. The amplitude of the laser output can also be affected by reflected light from the disc, making the laser output power noisy. Although some proposals have been made for removing the laser from the area of the head in certain apparatus (see, e.g., Lurie et al, "High Performance Optical Reader for Video Disc Substrates," RCA Review, Vol. 43, pp. 128-166 (March 1982)), several other serious problems remain. The most serious problem is the fact that each laser typically emits light in the form of a beam with an irregular amplitude distribution across the beam waist. This distribution is characterized by free space, spatial modes above the fundamental mode. When an attempt is made to focus such a waist, the image is typically a pattern which is not ideally suited for writing or reading unless a very tedious and expensive alignment process is performed between the optics in the head and the laser or very complex optical configurations are employed. Since each laser emits a different beam, this alignment has to be repeated for each apparatus and each time a new laser is installed in existing apparatus. A further problem with existing apparatus is the costly down-time which results if a laser should fail. Also, for certain systems, it may be desirable to provide additional power in the laser beams than is available from a single laser.
It is also recognized that the data rate for a given rate of disc rotation can be increased if several tracks are accessed simultaneously. Alternatively, a given data rate can be maintained while the disc rotation is decreased if it is desirable to minimize power needed to rotate the disc or simplify the design for the disc drive mechanism. Multiple track reading is usually thought to require integrated arrays of lasers and/or lenses (see, for example, U.S. patent application of Damen-Duguay-Howard-Jackel-Skocpol, Ser. No. 487,248, filed on Apr. 21, 1983 and assigned to the present assignee). While this is a viable approach to the problem, the difficulties associated with fabrication of arrays of identical lasers or lenses may or may not provide optimum cost effectiveness in the near future.
It is therefore an object of the invention to provide a read/write/tracking system which overcomes the problems enumerated above.